Consequently, the organisms are exposed to penetrating radiation from outside in doses fluctuating between 48 and 410 mrad/year, and, even reaching one rad/year and more in localities particularly rich in radio active substances. This, however, is not the only penetrating radiation to which the organisms are exposed. The radioactive isotopes disseminated in nature (see above) gradually accumulate in the various organs and tissues from the substances reaching the plants through soil and water, and the animals through the ingested plants or bodies of other animals, through their drinking water, as well as through the air. As a result of radioactive decay, the organisms are also irradiated from within in doses amounting to approximately 30 mrad/year.
These are the small doses of irradiation to which generations of orga nisms have been constantly exposed over hundreds of millions of years. Clearly, if such doses would have proved injurious for the living world, the latter would have either perished, or developed certain properties, in the course of evolution, that would protect it from the effects of background radiation. Since life on Earth has existed for a billion years, it has ob viously adapted somehow to the cosmic and planetary sources of penetrating radiations. Even places with exceptionally high natural radioactivity are inhabited by microbes, plants, and a variety of animals.
The study of radioactivity and of its potent and singular effects on various organisms began to develop by the turn of the century. Scientists naturally focused their attention more on ore dumps and on the already processed minerals, which contained considerably larger amounts of radio active isotopes than ordinary soil.
The Czech scientists Stoklasa and Penkava published in 1932 an ex haustive review concerning the effects of uranium and radium on organisms, and describing in the light of the research achievements at that time, the character and condition of wild plants growing on such dumps or in radium rich soils. The majority of such plants were reported to develop better and more rapidly than those in nearby plots with ordinary soils. However, unusual diseases were occasionally recorded, especially in perennial grasses, resulting in changed pigmentation of leaves, slower growth rate, and death. More recent publications indicate, although not very clearly, that in areas with high natural background radiation, plant communities may display certain properties manifest in the positive or negative selectivity of the individual plant species in such habitats. Phytogeologists
can readily establish the variations in the condition and density of repre sentatives of individual species, either from their unexpected occurrence and even predominance in the plant cover of a region, or from their complete absence in regions in which they apparently should have been in cluded in the local herbaceous cover, according to the general conditions, and in comparison with similar but nonradioactive neighboring regions.
From the modern point of view, the discovery of the striking effects on organisms of different nonradioactive microelements (i. e., substances found in soil in minute, almost indeterminable concentrations) may be attributed to the oligodynamic geochemical action of some nonradioactive elements associated with uranium and radium, rather than to penetrating radiations. The generalizations made by Stoklasa and Penkava, as well as other current generalizations, fail to take into account the biologically important fact that increased radioactivity in the biosphere is accompanied with increased concentration of free ions, including atmospheric gas ions (air ions) which are inhaled.
It is well-known that the exposure of radioactive ores to light and air during their mining and transportation to concentration plants leads to their waste through erosion and deflation, resulting in areas with high background radiation. Occasionally the ore dumps in concentration plants and the waste piles near factories producing radioactive preparations also raise the background radiation over large areas /8/. Convincing data /9, 10/ confirm that solid particles of radioactive aerosols contaminate ex tensive areas around working reactors. The radioactivity of water and the range of dispersion of radioisotopes discharged in water bodies together with the diluted effluents from nuclear and other industries and from scientific institutions working with radioisotopes have been reported repeatedly /11-18/. We now have fairly reliable data concerning global contamination with radioisotopes of the atmosphere, dry land, and oceans following experimental explosions of atomic and hydrogen bombs /10, 19, 20, 21, 22/, breakdowns of atomic reactors /10, 23/, etc.